1. Field of the Invention
The present invention relates to a multi-band image photographing method and apparatus, and a program for executing the method. In particular, the invention relates to a multi-band image photographing technique with which a multi-band image providing satisfactory noise balance to a reproduced image and contributing to reproduction of a natural image is obtained.
2. Description of the Related Art
In recent years, the progress of an image pickup technique is attaining commercialization of a multichannel photographing unit (multi-band camera), in particular, a photographing unit having plural channels (bands) that allow a spectral waveform of a subject to be restored with a practically sufficient accuracy. This is a camera with which a subject is photographed in plural wavelength regions (multi-band) that allow plural kinds (four or more kinds in many cases) of light to pass therethrough, thereby obtaining plural image data (spectral image data (multi-band image data) for each band (channel)). The camera is mainly constructed by, for example, a CCD and a color separation filter.
In the multi-band camera, reflected light obtained by illuminating a subject with illumination light having a specified (constant) spectral intensity distribution is allowed to pass through a spectral filter having a specified (constant) spectral transmittance distribution which is different for each band. The light is then focused on a light-receiving surface of an image pickup device via an optical lens of the camera having a specified spectral transmittance distribution, whereby a multi-band image having spectral images for respective plural bands is obtained.
Therefore, in the conventional multi-band camera, the sensitivity of each band is determined based on the product of the characteristics of spectral filters for respective plural bands and the spectral sensitivity distribution of an image pickup device.
That is, the effective exposure amount at the time of photographing with the multi-band camera is defined by the product of filtering characteristics of the spectral filters, that is, spectral transmittance distributions and a spectral intensity distribution of illumination light at the time of photographing. More precisely, the effective exposure amount is defined by including a spectral sensitivity distribution of the image pickup device in the product, and by further including a spectral transmittance distribution of the optical lens of the camera in the product.
However, deviation in these spectral characteristics may cause unbalance of the effective exposure amount for each band, and the dynamic range of a certain band may become narrow compared with those of other bands. As the result, there is a problem that an S/N ratio of the band decreases and data in the band is acquired as image data containing noise.
On the other hand, in the case where there is a deviation in the spectral distribution of a light source as exemplified by a fluorescent lamp, a deviation occurs in the intensity of the illumination light source in each band. As a result, image data obtained by photographing in a band having a low illumination light source intensity provides an image with lower S/N ratio than that in image data obtained by photographing in other bands. The effective exposure amount obtained by photographing in each band using a multi-band camera can be deemed to be given by a function of the “spectral sensitivity of a multi-band camera” corresponding to each band and the “spectral energy distribution of a light source” corresponding to each band. The amount of light obtained in each band and the S/N ratio has a functional relation in which the S/N ratio is improved together with the increase of the amount of light.
In this case, in a reproduced image obtained by compositing spectral images of the respective bands, noise or unevenness in a certain color becomes conspicuous, that is, decrease in image quality occurs.
An exemplary method of removing such noise includes a method in which a noise suppressing parameter is changed in accordance with the noise level by later image processing. However, there is a problem that such a method may cause differences in the artifact upon noise suppression if there are too large differences in the noise level and consequently lead to reproduction of an unnatural image.